The present invention relates to the field of optical circuits. More specifically, the present invention relates to a planar lightwave circuit (PLC) having reduced insertion loss.
Optical circuits such as PLC""s are used in a variety of applications. One application is in the area of communication systems. In these systems, a switching device receives a plurality of optical signals from input terminals. Then, these signals are selected, redirected, and transmitted to output terminals. Often, the redirection is performed using an array of switches in matrix layout.
In order to perform the switching functions, some switching devices first convert the optical signals into corresponding electrical signals. Next, the electrical signals are selected and redirected. Then, the electrical signals are converted back into corresponding optical signals for transmission to the output terminals. Such devices or components are often referred to as OEO (Optical-Electrical-Optical) devices. The OEO devices, because of the conversion requirements, require the use of expensive components, restrict the potential bandwidth of the data communication system, or both.
To overcome the shortcoming of the OEO devices, fully optical devices having no conversion requirements are used in some systems. In fully optical switching devices, optical signals are carried along a waveguide. Perturbations are formed within or along the waveguide to perform switching or other functions or operations on the optical signal traveling in the waveguide. Typically, the waveguides are laid out as a grid having intersections, or cross points, and the perturbations are formed at the intersections thereby creating a matrix of perturbations. For instance, a perturbation can be a liquid filled trench used as an optical switch, a doped portion of the waveguide, or other structure or material having a refractive index different than the refractive index of the waveguide. The refractive index of the switching perturbation is capable of being changed between a number of possible states, for instance between two levels. One of which causes light to pass through the perturbation without changing direction, and the other of which causes the light to change direction and pass into the crossing waveguide. In the case of a liquid filled trench, the liquid may be moved aside to leave a gaseous phase at the crosspoint, or a bubble may be created within it. In either case, the trench defines a three-dimensional index of refraction distribution, positioned at a cross-point of two waveguides, and changing its value performs the switching operation.
FIG. 1 illustrates a top view of a sample optical switching device 10 that is able to switch up to M input optical paths (designated Input-111,Input-213, . . . Input-M 15) to up to N output paths (designated Input-117, Input-219, . . . Input-M 21). In FIG. 1, the input paths, illustrated as rows, and the output paths, illustrated as columns, form a grid having M times N cross points. The input and output paths can be waveguide segments. While described herein as waveguide segments, the optical paths can be any optical paths capable of conducting an optical signal.
To perform the switching function, the device 10 includes an array of individual optical switches, one switch located at each cross point, or intersection. In FIG. 1, for convenience, these switches are designated as Si,j where i is the input row and j is the output column. FIG. 1 further illustrates two possible optical paths for example. The first path allows an input signal Input-1 to be redirected to an output terminal Output-2. The input signal Input-1 is illustrated by vector 12a. The input signal Input-112a passes through switch S1,1 but is redirected by switch S1,2 toward the output terminal Output-2. The redirected signal is illustrated by vector 12b. The second path is illustrated using input signal Input-2 (illustrated by vector 14a) being redirected toward an output terminal Output-N. The redirected signal is illustrated by vector 14b. The input signal Input-214a passes through switches S2,1 and S2,2 but is redirected by switch S2,N toward the output terminal Output-N. Such optical paths and switch configurations are known in the art. For example, see U.S. Pat. No. 5,699,462 granted to Fouquet, et al.
Various techniques of implementing the switches of the device 10 exist. For example, each of the switches of the device 10 can be implemented using mirrors, doping, liquid filled trenches, or other perturbation of the optical signal. For simplicity, the waveguides in FIG. 1 are shown intersecting each other at right angles. In this case, if the switches are implemented as liquid filled trenches, they would meet the input and output waveguides at 45 degrees. In practice, there may be good optical reasons for designing waveguides that intersect at an angle other than 90 degrees, in which case the trenches would be positioned at a correspondingly different angle with respect to the waveguides.
As an example, FIG. 2 illustrates a side view of a section 18 of the device 10 of FIG. 1 including a fluid optical switch S2,2 having a trench 22. The switch is defined by the trench 22 formed between a break in a waveguide 24 and other layers of the device 10 including, without limitation, cladding layers 26 and a heating circuit layer 28 including a heating element 29. All the layers of the device 10 is typically built on a substrate 30. Such switches are known in the art. The trench 22 is filled with gas or boilable liquid and is fabricated such that the trench 22 obliquely crosses the waveguide 26. Optical signals, for example, optical signal 14a, travel along the waveguide 26 and either crosses or is redirected by the switch S2,2 depending upon the state of the liquid in the trench 22.
Optical signal loss occurs at each stage of the transmission. This is called insertion loss. The insertion loss of a component or an optical path is normally defined as the difference between the power entering and leaving the component or optical path. The insertion loss limits the distance over which the signal can travel. In a device, such as an optical switch, the insertion loss limits the number of switches that can be effectively used to control or operate on input signals. Accordingly, there remains a continuing need for methods and apparatus to reduce and minimize the insertion losses in an optical switch and other optical components.
These needs are met by the present invention. According to one aspect of the present invention, a planar lightwave circuit (PLC) includes an array of waveguides intersecting at cross-points, each waveguide having a waveguide refractive index distribution for guiding an optical signal. The PLC further includes active perturbations for operating on the optical signal and passive perturbations having a refractive index distribution different than the waveguide refractive index distribution for reducing insertion loss of the PLC.
According to a second aspect of the present invention, an integrated optical circuit includes a waveguide for conducting optical signals and active perturbations for operating on the optical signal. Further, the circuit includes passive perturbations such that spacing between perturbations leads to reduction in insertion loss of the circuit.
According to a third aspect of the present invention, a planar lightwave circuit (PLC) has an array of waveguides intersecting at cross-points, each waveguide having waveguide refractive index distribution for guiding optical signal. Further, the PLC includes perturbations for operating on the optical signal, the perturbations spaced to minimize insertion loss of the PLC.
According to a fourth aspect of the present invention, a method of efficiently operating on optical signals within a waveguide of an integrated optical circuit is disclosed. Active perturbations operate on the optical signal to perform a function. The optical signal passes through passive perturbations. The active perturbations and the passive perturbations have a predetermined spacing whereby insertion loss is reduced.
According to a fifth aspect of the present invention, a method of fabricating an integrated optical circuit including a waveguide for conducting optical signal is disclosed. The circuit is fabricate having perturbations with predetermined spacing thereby reducing insertion loss.
According to a sixth aspect of the present invention, a planar lightwave circuit includes an array of waveguides which intersect at crosspoints, each waveguide having a three dimensional refractive index distribution for guiding light. The circuit further includes a plurality of perturbations positioned within the array, each perturbation having a three dimensional refractive index distribution different than that of each waveguide, and operative to reduce optical insertion loss.